Каждый h3 тэг с новым размером шрифта

var h3 = document.querySelectorAll('h3'); 
 
h3.forEach(function (el, i) {  
  el.style.fontSize = `${(i + 1) * 16}px`; 
});

<script src="//ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<h3>Some Text 1</h3> 
<h3>Some Text 2</h3> 
<h3>Some Text 3</h3> 
<h3>Some Text 4</h3> 
<h3>Some Text 5</h3>

$fontSize: 16px; 
$heading: 5; 
@for $i from 1 through $heading { 
  h3:nth-of-type(#{$i}) { 
     font-size: #{$i * $fontSize} 
  } 
}

<h3>Some Text 1</h3> 
<h3>Some Text 2</h3> 
<h3>Some Text 3</h3> 
<h3>Some Text 4</h3> 
<h3>Some Text 5</h3>